1. FIELD OF THE INVENTION
The present invention relates to fuel fired furnaces. More specifically, the present invention relates to controls for the air circulating blower in a hot air furnace.
2. PRIOR ART
Many types of blower controls have previously been used to provide safe and efficient operation of a hot air furnace. Blower operation is optimally designed to allow the furnace heat exchanger to achieve operating temperatures prior to blower operation and to allow the heat exchanger to cool before blower operation is discontinued. An over-temperature limit switch in the event of high bonnet temperature is usually included as a safety feature to commence blower operation regardless of the temperature of the space to be heated should the bonnet temperature within the furnace reach a predetermined high level.
Typical examples of blower control circuits include U.S. Pat. Nos. 3,554,003, 3,587,558 and 3,664,414; all issued to Raleigh and assigned to the assignee hereof. These patents all include a temperature responsive switch for actuating the blower motor. Other United States Patents having temperature dependent relays controlling blower operation include: Kriechbaum U.S. Pat. No. 2,862,666; Coffman U.S. Pat. No. 3,776,214; Seifert U.S. Pat. No. 2,801,055; and Reichelderfer U.S. Pat. No. 2,704,571. In U.S. Pat. No. 2,615,634, issued to Harbin, there is disclosed a manual switch in combination with a variable speed relay for controlling a furnace blower motor.
None of this cited art discloses the use of normally closed relay contacts within the blower motor circuit controlled by a time-delay circuit. The advantages of this control method include blower operation in the event of a control circuit failure such as loss of current or voltage within the control circuit or the blower relay; continuous blower operation upon failure will signal malfunction to the operator; and, blower response is immediate since there are no thermal delays in the system.